1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus for an internal combustion engine and an exhaust emission purifying method of the same. More particularly, the invention relates to a technique for enabling a reduction catalyst to avoid any thermal permanent deterioration caused by the exhaust gas which has passed through a particulate filter in an exhaust emission purifying apparatus including such particulate filter arranged for removing particulate matters in the exhaust gas exhausted from an internal combustion engine and a reduction catalyst arranged for purifying nitrogen oxides (“NOx”, hereinafter) in the exhaust gas.
2. Description of the Related Art
There is known to provide an apparatus for purifying particulate matters and NOx included in the exhaust gas from an internal combustion engine. In the apparatus, a particulate filter for removing particulate matters in the exhaust gas is disposed in an exhaust passage, and a reduction catalyst for purifying NOx is disposed downstream of the particulate filter (refer to Japanese Unexamined Patent Application Publication No. 2003-184542).
However, this known exhaust emission purifying apparatus for an internal combustion engine has some problems as described below. Namely, when particulate matters on the particulate filter are spontaneously ignited and burned, the temperature of the exhaust gas increases in the particulate filter, and the high temperature exhaust gas flows into the reduction catalyst disposed downstream, thereby thermally degrading the reduction catalyst. It is known that the thermal degradation of the reduction catalyst results in unrecoverable permanent deterioration. Conventionally, concerning an exhaust emission purifying apparatus for an internal combustion engine including a particulate filter, in order to avoid clogging of the particulate filter caused by excessive deposition of particulate matters, it is known that either an injection timing of fuel is adjusted to increase the temperature of the exhaust gas, or the exhaust gas is heated by an electric heater or a burner to periodically burn deposited particulate matters so as to regenerate the particulate filter. The degradation of the reduction catalyst caused by inflow of the high temperature exhaust gas occurs not only in case of the spontaneous ignition, but also when particulate matters on the particulate filter are intentionally burned for the regeneration purpose.